1. Field of the Invention
The present invention relates to a frequency discriminator, in a multiple quadrature amplitude modulation (QAM) system, for detecting a difference between a carrier frequency for a received signal and a reference carrier frequency for use in demodulation, and to a phase angle detector, provided in the frequency discriminator, for detecting a phase angle of a received signal on a phase plane, and in particular to a phase angle detector having a simple structure which can detect the direction and the degree of the phase angle of a received signal and to a frequency discriminator employing such a phase angle detector.
2. Related Arts
As a consequence of recent discussions concerning the employment of multi-digital channels for cable televisions, the use of 16 QAM and 256 QAM systems for this purpose has become common. In the multiple quadrature amplitude modulation system, 2.sup.n types of signs correspond to 2.sup.n signal points, and sign information available at a signal point is transmitted by employing an I channel signal (hereinafter referred to simply as an I signal), which corresponds to the real numerical value of the signal point, and a Q channel signal (hereinafter referred to simply as a Q signal), which corresponds to an imaginary number. The I signal and the Q signal are multiplied by carrier signals whose phases are shifted 90.degree., and the thus obtained signals are added together. The resultant signal is transmitted after its frequency has been raised to that of a carrier frequency required for transmission.
On the reception side, the received signal is down-converted into a symbol rate and the resultant signal is multiplied by a reference carrier signal, whose phase is shifted 90.degree., to demodulate the I signal and the Q signal in a baseband. Since on the reception side an oscillator for generating a reference carrier signal fluctuates due to manufacturing variances or as a result of the elapse of time, a carrier reproduction circuit called a carrier recovery circuit is provided in a receiver to generate a carrier reference signal having the same frequency as a carrier signal on the transmission side.
The carrier recovery circuit also captures the frequency of a reference carrier signal in order to eliminate the difference between the phase of a received signal at the signal point and the phase of the signal at the original signal point. However, in the multiple quadrature amplitude modulation system, since the intervals at which the signal points are positioned in space are small, the capture range for a PLL circuit in the carrier recovery circuit is so small, and the recovery of an accurate reference frequency is required at the time of demodulation. A frequency discriminator for detecting the difference between the carrier frequency of a received signal and a reference carrier frequency is employed as means for enlarging the capture range of the carrier recovery circuit. An obtained frequency difference is used as a control reference for the carrier recovery circuit, and detection of a frequency difference across a wider range is enabled.
FIG. 1 is a diagram illustrating an example arrangement of signal points on the phase plane for the 64 QAM. As is shown in FIG. 1, 64 signal points (black dots) are located on the phase plane which consists of a real I channel axis side and an imaginary Q channel axis side. A 6-bit sign string is allocated for these 64 signal points. The coordinates along the I axis and the Q axis corresponding to an allocated signal point are transmitted as an I signal and a Q signal.
When the frequency of the reference carrier on the reception side is shifted from the carrier frequency of the received signal, a demodulated signal point is rotated clockwise or counterclockwise. When, for example, a carrier frequency Fin of a received signal is higher than reference carrier frequency Flo for demodulation, a sequentially demodulated signal point is rotated counterclockwise from signal point P1 to P2, for example. Therefore, a difference .DELTA.f between the two carrier frequencies can be acquired by detecting the phase angle between the sequential signal points. When the two carrier frequencies Fin and Flo match, a signal point is not moved.
FIG. 2 is a diagram illustrating a conventional frequency discriminator. In FIG. 2, input to an I channel input terminal and a Q channel input terminal are an I signal and a Q signal, which are obtained by multiplying a received signal by demodulation reference carrier signals, for an I channel and a Q channel, whose phases are shifted 90.degree.. A phase angle detector 10 calculates tan.sup.-1 (Q/I) by using the I signal and the Q signal to obtain phase th1 for a demodulated signal point. A phase difference detector 14 calculates phase difference dth for sequentially demodulated signal points. The phase difference detector 14 comprises: a delay flip-flop 15 for delaying the phase th1 for the signal point (.theta. in FIG. 1) by using a demodulation reference clock; an adder 16 for calculating a difference between the phases th1 and th2 for the sequential signal points; and a converter 17 for calculating an absolute phase difference dth by using the output th2-th1 of the adder 16.
The frequency discriminator further includes an amplitude detector 12 for acquiring an amplitude d1 for a demodulated signal point from the I signal and the Q signal, and a signal point 18 for detecting an event when signal points outside a mask circle S shown in FIG. 1 are sequentially demodulated. The amplitude detector 12 performs an arithmetic operation (I.sup.2 +Q.sup.2).sup.1/2 to obtain the amplitude d1 for the signal point, and a comparator 19 compares the amplitude d1 with the Im radius of the mask circle S. When d1&gt;Im, output d2 goes to level H, and when signal points outside the mask circle S are sequentially received, output d4 is raised to level H by the delay flip-flop 20 and an AND gate 21.
In response to a detection signal d4, a complete integration circuit 22 accumulates the phase difference dth. The complete integration circuit 22 includes an adder 23 and a flip-flop 24 for latching the output of the adder 23 when the detection signal d4 is at level H.
As is described above, when signal points outside the mask circle S are sequentially detected, the conventional frequency discriminator calculates phase differences between the sequential signal points and adds them together. Since the signal point is moved counterclockwise or clockwise in accordance with the difference between the carrier frequency of a received signal and a reference carrier frequency, the frequency difference can be calculated by using the sum of the phase differences dth, and can be output, including its shift direction, to the output terminal OUT.
However, to constitute the frequency discriminator in FIG. 2 by using a common logic circuit, the phase angle detector 10, which calculates the phase angle for a signal point on the phase plane, requires an arithmetic operation circuit to perform the calculation for tan.sup.-1 (Q/I). A logic circuit for performing such a complicated calculation as that required for tan.sup.-1 (Q/I) is large, and therefore, is not appropriate for integration. Even when a lookup table is employed for the calculation results, rather than the complicated operation circuit, for the table a large expenditure for chips is required, and still the problem is not resolved.